Use better exception for AUTH_USER_MODEL

If AUTH_USER_MODEL is improperly configured as 'project.customer.User',
the error is:

ValueError: too many values to unpack

Use rather standard Django's error:

ImproperlyConfigured: AUTH_USER_MODEL must be of the form
'app_label.model_name'
Merge pull request #904 from tangentlabs/issue/custom-user-model-error

Use better exception for AUTH_USER_MODEL